In the history of computer systems, highly centralized mainframe computers initially dominated the industry. The mainframes later lead the way to the less expensive and more versatile minicomputers. With the advent of the personal computers, computing power was distributed to end users at the desktop. This paved the way for the powerful and high-speed workstations. Ultimately, these personal computers and workstations were connected together in order to allow the end users to share files, applications, and peripheral hardware (e.g. printers, storage devices such as disk drives, etc.). Thereby, individuals and work groups could share information and expensive computing resources while increasing the reliability of the computer system. This type of arrangement came to be known as Local Area Networks (LANs).
Basically, a LAN is comprised of a number of data terminals or data terminal equipment (DTE) which are coupled to transmission lines (i.e. circuits, channels, or trunks) through transceivers. A DTE is an individual device such as a personal computer, a workstation, a mainframe computer, a dumb or intelligent terminal, etc. A transceiver couples a DTE to a transmission line and performs any necessary signal conversions. The transmission lines conduct bits of data between the interconnected DTEs. Often, a concentrator (i.e. an intelligent hub or a hub) is used to route and manage traffic on the network.
Various network configurations (topologies) evolved. Some of the more popular configurations include the star, ring, tree, and bus topologies. In addition, various protocols for these networks also evolved. The most widely used protocols include the token bus, token ring, and Carrier Sense Multiple Access with Collision Detection (CSMA/CD) which is sometimes referred to as Ethernet.
Because of the numerous advantages offered by LANs, their numbers and sizes increased. Eventually, individual LANs were interconnected to form expansive, enterprise-wide networks. A collection of computer network entities came to be known as a domain. Repeaters were used to copy and forward bits of data from one network to another, simulating one large network from the combination of two or more separate but similar networks. In cases wherein networks had different protocols (e.g. Ethernet vs. Token Bus), bridges were implemented as an interface so that these networks could become interconnected. Bridges are smart, software-intensive devices. Bridges can be programmed to listen to all network traffic, examine each data packer's destination, and selectively forward each data packet while making any necessary changes. For interconnecting two different networks (e.g. Token Bus vs. X.25), a router is used. Routers also have the capability of directing data packets to their final destination by the least costly available path. Other devices and circuits have been implemented so that various LANs having different cabling schemes, access methods, protocols, operating systems, applications and computing devices can all be interconnected to form one vast network system.
When users wish to interconnect together two or more hitherto independent computer networks, it would be convenient if the interface could be performed automatically by the computer network. All the user would be required to do is to simply physically connect the cables to the proper network ports. The rest of the interfacing would be handled internal to the computer network and transparent to the user.
In the past, a wheatstone-bridge based implementation was used to detect the presence of whether a cable was connected to a particular port. However, this implementation required a 12 volt power supply. In some applications, a 12 volt power supply is not readily available. Furthermore, this wheatstone-bridge approach requires the use of special customized magnetics, which is relatively expensive.
Another prior art approach for detecting the presence of a cable connected to a port involves the use of a phantom signaling scheme. In this scheme, one of the concentrators sends out a predetermined "phantom" signal out of that port. If that port happens to be in use, a reply signal will be generated as an acknowledgment to the sent phantom signal. This scheme works fine so long as all of the concentrators belonging to that network recognize and support the same phantom signaling scheme. If a user wishes to interconnect computer networks having concentrators manufactured by different vendors, one of the concentrators might have an incompatible phantom signaling scheme or might not even have the a phantom signaling capability.
Thus, there is a need in the prior art for an inexpensive, efficient, and reliable mechanism for monitoring the presence of cables connected to ports of a computer network. Such a mechanism must be non-intrusive and cannot interfere with the operation of equipment connected to those ports.